A two dimensional optical image typically is captured electronically as a rectangular array of samples called picture elements abbreviated as pixels or pels. One may envision the pels arranged by horizontal rows and vertical columns. Charge coupled device (CCD) arrays are used to provide electrical representations of such optical images by generating charge packets corresponding to the light intensity of pels of the image. In general, the charge packets representing a row of pels are simultaneously transferred in parallel to a serial CCD register which shifts them serially to an output device. The output device then converts these charge packets in sequence to an electrical output signal.
The image representation produced by a CCD array may be enhanced by computing the second derivative, or Laplacian, for the picture elements of the image. The Laplacian may be used for such tasks as dynamic thresholding, edge detection and edge enhancement. In a book entitled Digital Picture Processing, by Aziel Rosenfeld and Avinash C. Kak, at Section 6.4, pages 179-191, it is indicated that the second derivative, or Laplacian, for each picture element of a light image may be approximated by computing the difference between the light intensity measured at each picture element and the average light intensity measured in an area surrounding the picture element.
An equivalent number of area averages and picture elements are desirable, so that areas being averaged may overlap, and each pel may contribute to several images. Alternatively the imaging capability of a CCD array may be enhanced by generating area averages about certain spaced-apart individual pels of an image, wherein areas being averaged are contiguous rather than overlapping and each pel contributes to only one average. Such average signal data plays an important role in performing various image processing functions. These functions may include dynamic thresholding, filtering, edge enhancement, and digital half-tone.
One way to achieve area averaging is to digitally store a sufficient number of scan lines of the output signal of the imager, so that these stored values can be read out and processed by a digital processing unit. However, such a method is costly and is limited in speed by the performance of the digital hardware and software. Moreover, the arithmetic operation to find the mean of a set of numbers on a processor requires many Central Processing Unit cycles and memory cycles and, so, may be quite time consuming. The situation is more severe for high speed and high resolution scanner applications, since once the scanning operation of such a device is initiated, it usually cannot be interrupted due to electrical or mechanical reasons. Therefore, it may be necessary to store data representing the entire scanned image for processing purposes.
In prior art image scanners incorporating Laplacian computation, the Laplacian for each picture element of an image is derived by irradiating an ordinary CCD array with a focused image and an unfocused image. The focused image results in the formation of charge packets which are proportional to the focused light intensity at the picture elements of the image. The unfocused image forms charge packets which are proportional to the average light intensity around each picture element. The Laplacian is then derived for each picture element by subtracting the unfocused charge packets from the corresponding focused charge packets.
The equipment which has been used to accomplish this task may be fairly expensive and complicated. In order to generate the data, the apparatus may either split an image into focused and unfocused components to irradiate separate CCD arrays or may optomechanically switch a focused and unfocused image to irradiate a single CCD array.
A more efficient means of computing the Laplacian is disclosed in a paper, "A Multiple Output CCD Imager for Imaging Processing Applications", by J. E. Hall, J. F. Brietzmann, M. M. Blouke, J. T. Carlo, Int. Electron Devices Meeting Tech. Digest, Washington, D.C., Dec. 4-6, 1978, pp. 415-418. The disclosed imaging device includes an imaging array and a peripherally placed serial register that is employed to serially gate line charge packets from the imaging array. Charge packets at particular points of successively imaged lines are tapped off at fixed points on the serial register to be summed, thus computing area averages for the picture elements of the image.
Only the means of delaying, storing and tapping off are provided, leaving the summing to be performed in other devices. The apparatus has the further disadvantage that transfer inefficiencies in the serial gating process tend to degrade the electrical representation of the image in proportion to the number of transfers that are executed. Accordingly, the accuracy of the serial transfer apparatus decreases as the number of picture elements in the line of an image increases. Thus, the accuracy of the apparatus is significantly reduced for images having a relatively large number of picture elements, for example, on the order of 2000 pels per line. Also, the length of the serial register and the number of output taps increase in proportion to the number of pels that are required to compute an area average. Due to transfer inefficiencies, the increased register length results in decreased accuracy.
Another means of computing the Laplacian is disclosed in J. M. White U.S. Pat. No. 4,264,930. Within a TDI imaging array, charge packets are generated representing the vertical sum of the pels within the area surrounding each pel. Each row of vertical sum charge packets is then shifted by a serial CCD register through a horizontal summing device which sums the vertical sum packets to produce area sum packets. The White apparatus has two drawbacks. First, the vertical sum is in an array of bucket-brigade charge replicators. As compared to charge coupled devices, bucket-brigade devices are known to be less accurate and more prone to interference from excess electrical noise. Second, the horizontal sum is performed serially. Thus, the rate at which the apparatus can capture and process an image is limited by the need to process thousands of horizontal sums in sequence.